My invention comprises improvements in a planetary gear transmission of the kind shown in my U.S. Pat. No. 5,073,156. That patent discloses a hydrokinetic torque converter and a compound planetary gear unit arranged in a transmission system for an automotive vehicle. The planetary gear unit is a so-called Ravagineaux-type planetary gear unit similar to the one that is described in U.S. Pat. No. 4,934,216. Both U.S. Pat. Nos. 5,073,156 and 4,934,216 are assigned to the assignee of this invention.
In both of the transmissions described in the prior art references described above, the planetary gear unit includes two torque input sun gears and a compound carrier assembly that includes long planet pinions meshing with short planet pinions. The long planet pinions engage a ring gear which in turn is connected to a driven shaft. The long planet pinions also engage the reverse drive sun gear with a larger pitch diameter. The short planet pinions engage the forward drive sun gear with the smaller pitch diameter and the long planet pinions.
In the transmission of the '156 patent, torque is distributed through a forward clutch to the smaller of the two sun gears during each of three forward driving ratios. A fourth overdrive ratio is achieved by anchoring the larger diameter sun gear and by clutching the compound carrier assembly to the torque input shaft. This overdrives the ring gear as the sun gear acts as a reaction point.
The clutch that is used during operation in the overdrive ratio is also the clutch that is used to establish direct drive operation in the third ratio. When that clutch is applied together with a companion clutch, which serves as a torque input friction element for the smaller of the two sun gears, all of the elements of the gear unit are locked together for rotation in unison.
On a ratio change from the third ratio to the fourth ratio, an overrunning clutch in series relationship with respect to the direct drive clutch free-wheels, thus effecting a nonsynchronous 3-4 upshift. Conversely, a nonsynchronous 4-3 downshift is achieved as the reaction member for the larger of the two sun gears is released.
The direct drive clutch disclosed in the '156 patent comprises a series of friction discs, a subset of the series of friction discs being connected drivably to the compound carrier and a companion subset being connected to the driving shaft through the forward clutch in series relationship with an overrunning coupling. Thus, on a 3-4 upshift, the overrunning coupling will free-wheel, thus establishing a nonsynchronous ratio change. At the time of that upshift, however, the carrier of necessity is rotating at an angular velocity that differs from the angular velocity of the forward clutch. Thus, the calibration of the direct clutch becomes complex because the dynamic considerations that are appropriate for one subset of friction discs is different than the dynamic considerations involved in the application of the friction discs of the other subset due to their differential rotary velocity. Also, a large number of friction discs is required in the arrangement of the kind shown in the '156 patent because the torque that is necessary to drive the compound carrier is delivered through only the discs of one subset of discs. Further, the torque associated with the smaller of the two sun gears is delivered to that sun gear only through the other subset of friction discs. Because of this arrangement, a large number of friction discs is required for any given torque capacity for the transmission.
The improvements of my invention make it possible to eliminate the differential angular velocity of the discs upon application of the direct drive clutch. This results in an improvement in shift quality on 3-4 upshifts and on 4-3 downshifts because differences in inertia torques are minimized. It also makes it possible to reduce the number of discs for any given torque capacity of the transmission. This in turn results in an economy of space and makes it more feasible to package the torque transfer elements of the gearing in a compact vehicle transmission installation.